The use of clamps for the handling of metal sheets, of plates made of other materials and similar pieces of reduced thickness, is known, these clamps comprising a clevis-shaped body equipped with means for attaching to a handling apparatus, such as a travelling crane, the clevis having one fixed arm and one pivotable arm shaped as a cam, actuated by a return spring in a mechanism shiftably past a dead point. The securing of the sheet inside the clamp is a result of a jamming effect of the two jaws of the clamp.
After the clamping of a sheet or the like, it is common to manually block the clamp in the locked position and to manually unlock the clamp, prior to opening it. Therefore, such clamps are very cumbersome in use.
It is also known to control the closing movement of the mobile jaw by means of a trigger arranged across the opening of the clevis and which is actuated by one edge of the sheet, when the jaw engages thereon. This pivoting trigger normally maintains the mobile jaw in an open position, which means in the ready-to-act position of the clamp.
When the clamp is brought into contact with the metal sheet, the pressure of the metal sheet against the trigger causes the latter to pivot, releasing the mobile jaw, which comes to rest against the sheet, by pivoting under the action of a spring. As a result, because of the shape of the cam constituting the mobile jaw, a self-locking of the sheet is produced when a traction is exerted upon the clamp. This technique is interesting, since it makes possible the automatic closing of the clamp when it comes in contact with the piece to be seized.
In order to open such a clamp, it suffices to actuate a lever associated with the mobile jaw, returning it to the open position, a position in which it normally has to be held by the trigger. However, when the opening of the clamp is effected by the pressure of the sheet against the trigger, the latter cannot maintain the pivoting jaw in the open position.
It is thus convenient simultaneously to effect the opening of the clamp and its disengagement from the sheet, in order to allow the trigger to fulfill its part as a locking member in keeping the mobile jaw in the open position. Such an operation is cumbersome to the user and requires great skill.